1. Field of the Invention
The present invention relates to a switching system between the working transmission lines and a protection transmission line, and more particularly, to a switching system for switching, the signals of higher signal level from either one of the working transmission lines to the protection transmission line if a fault occurs on the former transmission line.
A known switching system is set up with the protection transmission line that is commonly used by a plurality of working transmission lines. In the case where maintenance, fault restoration or testing is required with respect to one of the working transmission lines, switching is performed from the working transmission line to the protection transmission line, whereby communication through the working transmission line can be continuously maintained.
2. Description of the Related Art
According to a typical and conventional switching system, as will be explained hereinafter, the protection transmission line is not activated during a normal state, but is activated only when a fault restoration and so on is needed for some working transmission line, i.e., an abnormal state. Further, a dummy signal generator is required to periodically monitor the condition of the protection transmission line, because the protection transmission line is not frequently used. Thus, a communication network that operates on the above switching system does not work with high efficiency, since the protection transmission line is used most of time to transmit the dummy signal, which is irrelevant to the inherent communication of user information.
To overcome the above shortcomings of the above switching system, an improved switching system has been proposed and will be explained hereinafter. According to the improved switching system, an optional transmission line is incorporated in the switching system. During a normal state, protection equipment can transmit user information over its protection transmission line, user information which is given via the optional transmission line. In this case, the user information on the optional transmission line should be low in priority compared to the user information on the inherent working transmission lines. Thus, the aforesaid dummy signal generation can be eliminated and further the protection transmission line can be utilized to realize a higher performance communication network.
However, some problems arise in the above improved switching system. First, means for multiplexing the optional user information signal to the higher signal level is needed, because the protection transmission line, e.g., an optical transmission line, carries multiplexed signals of higher signal levels. Second, a protection switch mounted in the protection equipment must operate at very high speed, e.g., 600 Mbits/s, because the protection switch must switch when switching is required for a working transmission line, and a high speed optional user information signal to the user information signal on the related working transmission line is required which is a multiplexed signal of a high signal level. This means that the protection switch should be fabricated using high speed operating devices at a rate of, e.g., 600 Mbits/s, and thus the protection switch cannot be obtained at a low price because such high speed operating devices cannot be fabricated using commercially available and low cost complementary metal oxide semiconductor (CMOS) devices. Third, particular connectors are needed to join the multiplexing means and the protection equipment, the connectors of which cannot be manufactured without a special order.